2:00 PM - 2:15 PM
[HTT23-14] Nd isotopic variation of seawater along the Pacific coast of Tohoku district and its causal factor
Keywords:Nd isotope ratio, seawater
The Nd isotope ratios of seawater samples, which were taken from 14 coastal sites located between 38.17N and 40.55N, vary from -8 to +1 in εNd. These values are well correlated (r=0.72) with Nd isotope ratios of river water samples taken from the adjacent river of each coastal site, which ranges from -8 to 2 in εNd . This indicates that the εNd of coastal seawater is largely defined by the value of land water. On the other hand, the range of Sr isotope ratios of seawater samples is quite narrow and high (0.70916 to 0.70919) despite that the value of river water ranges widely from 0.7055 to 0.7085. The Nd isotope ratios of seawater and water of the adjacent river differ from each other in most sites although they are correlated significantly. Nd contribution from offshore seawater should be considerable. Mixing of river water and offshore seawater of Tohoku district (-4 to -3 in εNd; Amakawa et al, 2004) is consistent with the linear regression expression of εNd of seawater to that of river water (y=0.65x-0.83).
Contribution of land water is suggested to be limited to nearshore regions. Surface seawater samples were taken along an offshore-directed line from the Abukuma River mouth in December 2015 (winter) and July 2016 (summer). Nd isotope ratios in summer were almost uniform around -9 in εNd in the interval between 2km and 60km from the shore, whereas in winter it increases offshore from -6 to -2 in the interval between 35 km to 60 km from the shore. The εNd of about -9 is considered to be that of the Kuroshio Current (Amakawa et al., 2004), which dominates in summer at the region, whereas that of -2 is consistent with the value of the Oyashio Current (Amakawa et al., 2004). On the other hand, the εNd of the Abukuma River water (-2.9) is not reflected even at the site only 2 km apart from the shore. In spite of this regional limitation, Nd isotope ratio can newly provide a tool to trace animals and products at least in coastal seas.